ath6kl: fix regression in ath6kl_upload_board_file()

My patch 24fc32b3 ("ath6kl: add ath6kl_bmi_write_hi32()") caused a regression
in ath6kl_upload_board_file() and the board_address variable was not
properly initialised in some cases:

ath6kl/init.c:1068:6: warning: ‘board_address’ may be used uninitialized
in this function

Most likely this broke ar6004 support but I can't test that right now.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>